1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a double layered structure.
2. Description of the Prior Art
In recent years, there have been commercial developments of tapes wherein an alloy powder or metal powder is used as magnetic powder for high performance audio recording tapes. However, such a fine metal powder is very active in contrast with an iron oxide powder. In an extreme case, when exposed in air, such a metal powder undergoes spontaneous ignition. Accordingly, it is an important subject to provide a some safety measure. In general, the finner the metal powder, the more active it becomes. It is well known that if the metal powder is made coarse, it will be stabilized. However, the noise (AC bias noise) characteristics will be deteriorated by an increase of the particle size. Under the circumstances, it is obliged to use fine particles with a specific surface area of at least 30 m.sup.2 /g as measured by BET method. It would be possible to reduce the danger during the process for the production of the tape and also to minimize a change with time of the tape itself if the particle size could be enlarged to some extent so that a stabilized metal powder would be employed. In this respect, the present inventors have made a study to find out whether or not it is possible to use a magnetic metal having a more or less large particle size and yet to adequately reduce the noise.
On the other hand, an iron oxide-type magnetic powder in the form of fine particles is widely used for magnetic tapes. The iron oxide-type magnetic powder is chemically stable and gives a low noise level. However, the transfer characteristics and the low frequency sensitivity tend to deteriorate as the fine particles become finner. Accordingly, it has been difficult to employ a super fine particulate powder.